In the chemical process industry, where corrosive liquids, liquors and/or slurries are treated and transported through various kinds of piping, specially designed valves are employed for controlling the flow of these fluid materials through the piping. Depending on size and material content, a single valve can cost many thousands of dollars. Many of the liquids carried by the piping contain dissolved materials, which, under certain conditions, form scale. Scale formations are highly undesirable in flow control valves because significant amounts of scale buildup on valve seats, as well as around the moving parts of the valve ultimately lead to valve malfunction and failure. Formation of scale in valve seats prevents total closure of the valves with resultant leakage, and this leakage is particularly pronounced when the liquid is under pressure. Scale-encrusted valve discs and other movable valve parts can lead to the freezing of the valve mechanisms and complete operational failure of the valves.
Many attempts have been made in the past to design efficient self-grinding valves, which, either during seating and/or opening, perform seat grinding operations. This grinding action, among other things, cleans the valve seat, allows full seating of the sealing disc and/or allows raising of the scale-encrusted valve discs by freeing the disc from the seat through grinding. The valves described in the following prior art patents are representative of the self-grinding valves developed in the past to accomplish the above-described grinding operations: U.S. Pat. Nos. 1,991,621 (Noll) and 2,996,075 (Deimer, et al.), 3,071,149 (Rhodes), 3,220,431 (Morrell) and 3,311,121 (Morrell). The valves of these patents have not been satisfactory for a number of reasons, such as excessive costs, high maintenance or high torque requirements. U.S. Pat. No. 2,996,075, for example, shows a self-grinding valve wherein a mechanism is provided which allows for a grinding movement or rotation of the valve disc through a rotating valve stem. The nonrotational seating movement is accomplished by locking of the stem against rotation and application of a high closing thrust. The mechanism which controls this selective operation involves a torque converter consisting of a horseshoe-shaped pin which the operator can insert in or remove from the valve stem depending on the type of operation desired. Although the mechanism described provides a valve seat cleaning action, the construction and operation of this type of valve is complicated and no provision is made to protect the internal moving parts of the valve against the corrosive and scaling action of the fluids that contact these valve parts.
U.S. Pat. No. 3,071,149 likewise describes a self-grinding valve, which, while capable of cleaning the valve seat by grinding, fails to provide corrosion protection for the moving parts or the necessary torque required either to close or open the valve. Morrell U.S. Pat. Nos. 3,220,431 and 3,311,121 disclose complex and hard to maintain self-grinding valves wherein the linear movement of the valve stem is effected by a drive means which is separate from the drive means required for the normal rotational or grinding movement of the valve disc.
The valve of the instant invention is believed to constitute a substantial improvement over prior art valves, including those of U.S. Pat. Nos. 1,113,642; 1,551,436 and 1,736,253, in that it is of relatively simple construction, operation and maintenance. It is capable of being readily connected to hydraulic or electric motors. The valve includes a unique arrangement for coupling and uncoupling the valve disc relative to the main valve stem to effect disengagement of the same at the end of the grinding cycle so that the maximum force can be exerted upon this disc for seating purposes.
In the instant design, the principal moving parts of the valve, such as the valve stem and the internal parts of the valve disc are effectively shielded from the scale-forming material being handled so as to noticeably prolong the life of the valve without at the same time decreasing the overall efficiency thereof.
The instant valve is particularly useful in the processing of alumina-containing caustic solutions generated in the Bayer process treatment of bauxite ores wherein these materials are subjected to multiple pumping and valve controlled operations and buildups of hydrated alumina, alkali or silica scale quickly take place on the wetted surfaces of the valves, including, in particular, the valve seats, unless extensive and expensive maintenance practices are employed.